For High-Jumping Fleas, the Secret's in the Toes

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Fleas perform an amazing feat when they jump, and the mechanics
behind the tiny, bloodsucking pests' acrobatics have been studied
-- and debated -- for a long time. But new research may have
settled one major question: How do fleas lift themselves off the
ground?

It turns out the tiny insects push off the ground using their
shins and feet.

"It's been known fleas are incredible jumpers for 1,000 years,"
said Gregory Sutton, a research fellow at the University of
Cambridge in England. "If you see fleas in your hand and you see
them jump and you realize how small they are, it doesn't take
much to realize these guys are catapulting themselves huge
numbers of body lengths," said Sutton, who along with a colleague
completed this most recent study of the mechanics of a flea's
jump, an athletic effort that catapults the fleas 50 to 100 times
their body length.

One mystery solved, another remains

Fleas jump more quickly and with more force than would be
possible using muscle. This mystery was solved in 1967, when
researcher Henry Bennet-Clark discovered that the fleas, using an
elastic pad made of a protein called resilin like a tensed
spring, release the pad to catapult themselves into the air.
[Image of Flea Resilin]

But this discovery gave rise to another debate that remained
unresolved: How did the fleas transfer the force from the spring
mechanism to the ground to lift off? Bennet-Clark believed a flea
pushed down through its tibia and tarsus (equivalent to a human
shin and foot or toe, respectively). Meanwhile, fellow flea
researcher Miriam Rothschild believed that fleas pushed off using
their trochantera (equivalent to human knees).

Sutton and study collaborator Malcolm Burrows, also of the
University of Cambridge, resolved the debate using high-speed
cameras, an electron microscope and computer modeling, plus 10
hedgehog fleas (donated by the Tiggywinkles Wildlife Hospital
Trust).

Before fleas take off on a jump, their knees appear to rest on
the ground, creating an important sticking point in the debate.

With the aid of the cameras, the researchers filmed the fleas
jumping 51 times. In 45 of those jumps, the fleas' feet and knees
were on the ground when the flea pushed off. However, in the
remaining six, the knees were both clear of the ground at that
time, indicating they did not transfer the force of the jump to
the ground. In all of the jumps, the feet touched the ground.

Pictures taken through the microscope revealed that spiny
gripping structures — which come in handy while bracing during
lift off — covered the fleas' feet and shins. The knees,
meanwhile, were smooth. And finally, the researchers used
computer models to run simulations of both jumping theories.
Here, too, evidence supported the lower-leg push-off
theory.

"We pretty much discounted the hypothesis that force is going
through the knee," Sutton said. Their work appears in today's
(Feb. 10) issue of the Journal of Experimental Biology.

This doesn't mean there aren't more mysterious aspects to flea
jumping ripe for exploration. The team plans to look at how the
fleas control their jumps.

"We never observed any time when one leg would extend without the
other one, so we want to figure how the fleas do that," Sutton
said, explaining, "there doesn't appear to be any mechanical
connection between them, but they fire off at exactly the same
time."